Magnetic disk drives typically include a thin layer of lubricant applied to the surface of the recording disk, in order to reduce undesirable interactions between the head and the disk, such as friction and/or stiction. Typical lubricant layer thicknesses are in a range from about 0.8 nm to about 1.6 nm, but in some cases this range can run from 0.6 nm to 2.0 nm. Typically, the lubricant layer material is a perfluoropolyether (PFPE) lubricant. PFPE lubricants are copolymers with the following chemical structure:X—[(OCF2CF2)m-(OCF2)n]-O—X,where m/n=2/3 and X represents endgroups of the polymer. Through selection of these endgroups, the material is modified to vary the film's adsorption, affinity, reactivity, etc. Some common endgroups are X=F, X=CF2CH2OH and X=CH2OCH2CH(OH)CH2—OH for PFPE Z, Zdol and Ztetraol respectively. Some of these PFPE lubricants are also known under a variety of trademarks or trade names (e.g., “Fomblin Z” family, and “A20H” which is made from Zdol by Moresco).
As disk drive technology evolves toward increasing areal density and storage capacity, the separation between the head and the disk generally decreases. As a result of this decreasing separation between head and disk, undesirable interactions between the head and the lubricant layer become likely, and, in fact, have recently been reported (e.g., in Ma et al., IEEE Transactions on Magnetics, 38(1) pp 112–117, Jan. 2002).
The article by Ma et al. reports the formation of a periodic modulation of the lubricant thickness along a track. The amplitude of this modulation grows slowly as track flying time increases, where track flying time is the time spent by the head over a single track while the disk is rotating. The reported modulation has a period which corresponds to v/f, where v is the linear velocity of the track with respect to the head, and f is a mechanical resonance frequency of a head assembly including the head. More than one mechanical mode of the head assembly can contribute to lubricant thickness modulation. For example, in the Ma et al. article, a roll mode having a frequency of about 40–50 kHz and a pitch mode having a frequency of 196 kHz were both observed to contribute simultaneously to lubricant thickness modulation.
Such thickness modulation of the lubricant layer is undesirable, since it tends to drive the head assembly into unwanted motion. This problem is exacerbated by the fact that the perturbation induced by the lubricant thickness modulation is at one or more mechanical resonance frequencies of the head assembly, which tends to increase the effect of the perturbation due to resonant enhancement. This head motion is undesirable because it interferes with reading data from and/or writing data to the disk.
Accordingly, it is an object of the present invention to reduce the amplitude of such thickness variation of the lubricant layer, thereby reducing associated head motion and improving disk drive performance. A further object of the invention is to reduce the amplitude of such thickness variation of the lubricant layer using approaches which do not depend on reducing the physical interaction between head and disk (e.g., by increasing the separation between head and disk).